Treatment strategies in Rett syndrome so far have been essentially symptomatic and supportive. In order to establish the medium-term effects of L-carnitine, an open label trial was performed in a cohort of 21 Rett syndrome females, with a control group of 62 Rett syndrome females of a similar age, for a 6-month period. Compared with the Rett syndrome controls, treatment with L-carnitine led to significant improvements in sleep efficiency (P=0.027), especially in the subjects with a baseline sleep efficiency less than 90%, energy level (P<0.005) and communication skills (P=0.004). There was no significant difference in the subject's level of physical activity, hand function or in the quality of life of the subject's parents. In addition, before and after comparisons of the treatment group showed improvements in expressive speech (P=0.011). Treatment with L-carnitine seems to be of significant benefit in a subgroup of girls with Rett syndrome. In these girls, small but discernible improvements may be of considerable importance to their parents and carers.